1. Field of the Invention
The present invention relates to base station apparatuses, mobile stations, and communications control methods.
2. Description of the Related Art
Communications schemes to succeed W-CDMA and HSDPA (i.e., long term evolution (LTE)) are being studied in a W-CDMA standardization body called 3GPP. Of these communications schemes, OFDMA (orthogonal frequency division multiplexing access) is being considered for downlink, while SC-FDMA (single-carrier frequency division multiple access) is being considered for uplink as radio access schemes. (See 3GPP TR25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA,” June 2006, for example.)
The OFDMA, which is a scheme for dividing a frequency band into multiple narrow frequency bands (sub-carriers) and overlaying data onto the respective frequency bands to transmit the data, densely arranges the sub-carriers on the frequency axis such that one sub-carrier partially overlaps another sub-carrier without their interfering with each other, making it possible to achieve high-speed transmission and to improve frequency utilization efficiency.
The SC-FDMA is a transmission scheme which divides a frequency bandwidth and transmits using different frequency bands among multiple terminals to make it possible to reduce interference between the terminals. The SC-FDMA, which features a reduced variation in transmission power, makes it possible to achieve wide coverage as well as low power consumption of the terminals.
In general, in a mobile communications system, a random access channel is used in order to establish an initial connection in uplink. In other words, a mobile station transmits a random access channel to a base station apparatus when it starts conducting communications. The random access channel, which is also called a contention based channel, is a channel for requesting a resource allocation, etc. The random access channel is also used for purposes of initial connection establishment, handover, and uplink scheduling request, uplink synchronization establishment request, etc.
A preamble first transmitted from a mobile station in a random access procedure is, because of its nature, transmitted at an arbitrary timing by the mobile station, so that a base station apparatus is not able to know what timing the preamble is transmitted at. In this case, the base station apparatus estimates received power and interference power for all preamble sequences which may be transmitted, and detects the respective preamble sequences based on the received power and the interference power.
For example, as the W-CDMA uplink is a non-orthogonal system, the base station is able to estimate the interference power by calculating received levels of the whole bandwidth. A correlation is taken between all preamble sequences which may be transmitted and a replica sequence to calculate the received power.
On the other hand, a preamble sequence in the LTE sequence is generated using a Zadoff-Chu sequence, which is one of CAZAC (constant amplitude zero auto-correlation) sequences with a zero correlation zone, as defined in 3GPP TS 36.211 (V8.0.0), “Evolved Universal Terrestrial Radio Access (E-UTRA), Physical channels and modulation,” June 2006. For the Zadoff-Chu sequence, one sequence is used, or two or more sequences are used. In this case, preamble sequences generated from the same Zadoff-Chu sequence are orthogonal with one another, so that it is not possible to estimate interference power as in the W-CDMA system. For example, when all preamble sequences are generated from the same Zadoff-Chu sequence, even when a received level is high, signal powers of other preamble sequences are orthogonalized to become zero, so that an actual interference level may be small.